1. Technical Field of the Invention
The present invention relates to the field of current sense amplifiers.
2. Description of Related Art
Current sense amplifiers are typically used to measure the amount of current supplied by and to a device or component in various types of electronic equipment. Reference is now made to FIG. 1 wherein there is shown a schematic diagram of a typical implementation for a current sense amplifier 10. A current (referred to as Iload) to be sensed flows through a sense resistor (Rsense) from a first sensing node (Rs+) to a second sensing node (Rsxe2x88x92). The first sensing node (Rs+) is connected to a first input (+) of an operational amplifier 12 through a resistor R1. The second sensing node (Rsxe2x88x92) is connected to a second input (xe2x88x92) of the operational amplifier 12 through a resistor R2. The output of the operational amplifier 12 is connected to the base of a transistor 14 whose collector is connected to the first input (+). A current (referred to as Iref) flows through the connection between the first input (+) and the collector of the transistor 14. An emitter of the transmitter 14, generates an input current (referred to as Iin) and, is connected to an input of a 1:n current mirror 16. Vcc is connected to an output of the current mirror 16 through a load resistor R3 for the generation of an output current (referred to as Iout) The second input (xe2x88x92) of the operational amplifier 12 presents a relatively high impedance. Neglecting the presence of any input bias current at the second input (xe2x88x92), it is recognized that no current flows through the second resistor R2 from the second sensing node (Rsxe2x88x92) . The voltage at both the first input (+) and second input (xe2x88x92) is therefore equal to the voltage at the second sensing node (Rsxe2x88x92). The voltage drop across the resistor R1 is accordingly equal to the product of Iload and Rsense. The input current Iin thus equals the current Iref, and ideally then:
Iref=Iloadxc2x7(Rsense/R1);xe2x80x83xe2x80x83(1)
and
Iout=nxc2x7Iin;xe2x80x83xe2x80x83(2)
and
Iout=nxc2x7Iloadxc2x7(Rsense/R1).xe2x80x83xe2x80x83(3)
In practice, however, the value of base current at the transistor 14 cannot be neglected as it is also multiplied by a factor of n in the current mirror 16 and alters the value of the output current Iout (of Equation 3) away from ideal. Still further, it is recognized that a current mirror 16 possessing a large factor (for example, n equals approximately twenty for a Wilson current mirror) is not particularly accurate. The value of the input current is actually set as follows:
Iin=Irefxc2x7(1+1/xcex2)xe2x80x83xe2x80x83(4)
wherein: xcex2 is the current gain of the transistor 14.
Reference is now made to FIG. 2 wherein there is shown a schematic diagram of the 1:n current mirror 16. This current mirror has a conventional configuration that is well known to those skilled in the art. A detailed description of the components, interconnection and operation of the current mirror 16 is accordingly not required. Continuing with the foregoing analysis, and specifically with respect to the current mirror 16, the relationship between the input current Iin and the output current Iout is given by the following:                               I          out                =                              I            in                    ·                                                    n                ·                                  β                  2                                            +                              n                ·                β                            +              β                                                      β                2                            +                                                (                                      n                    +                    1                                    )                                ·                β                            +              n              +              1                                                          (        5        )            
wherein: xcex2 is the current gain of the matched transistors within the current mirror 16. From Equations 4 and 5, the actual value of the output current Iout is given by the following:                                           I            out                    =                      n            ·                          I              ref                        ·                          (                              1                -                                                                                                    (                                                  n                          -                                                      1                            n                                                    -                          1                                                )                                            ·                      β                                        +                                          (                                              n                        -                                                  1                          n                                                                    )                                                                                                  β                      2                                        +                                                                  (                                                  n                          +                          1                                                )                                            ·                      β                                        +                    n                    +                    1                                                              )                                      ;                  xe2x80x83                ⁢        and                            (        6        )            
by taking into account Equation (1);                               I          out                =                  n          ·                      I            load                    ·                      (                                          R                sense                            R1                        )                    ·                                    (                              1                -                                                                                                    (                                                  n                          -                                                      1                            n                                                    -                          1                                                )                                            ·                      β                                        +                                          (                                              n                        -                                                  1                          n                                                                    )                                                                                                  β                      2                                        +                                                                  (                                                  n                          +                          1                                                )                                            ·                      β                                        +                    n                    +                    1                                                              )                        .                                              (        7        )            
Now, from a comparison of the foregoing Equations, it is recognized that the actual output current (see, Equation 7) of the current mirror 16 and the current nxc2x7Iref that should preferably (and ideally) be output from the current mirror approximately differ from each other by a factor (shown inside the parenthetical of Equation 7) on the order of:       (          n      -              1        n            -      1        )    /      β    .  
Given a scenario where n is relatively large (for example, greater than or about ten) and xcex2 is relatively small (for example, less than or about sixty), this factor can present a significant difference in measured current. In this configuration, the current sense amplifier circuit of FIG. 1 cannot be used for generating a precision current amplifier output.
A transconductance amplifier measures a current passing through a sense resistor to generate a reference current indicative of the measured current. A current mirror circuit connected to the transconductance amplifier amplifies the reference current to generate an amplified output current. A cascode circuit is connected between the current mirror circuit and output of the generated amplified output current.